The objective of this research proposal is to establish some of the steps in the pathway for protein secretion from liver. We will use biochemical and genetic approaches in studies on rat hepatomas which make, but don't secrete, serum albumin to identify lesions in the secretory process. The albumin or its precursor forms which accumulate in these hepatomas will be characterized and compared to preproalbumin, proalbumin, and albumin produced by normal rat liver. In other experiments we will compare the membrane components, which appear to be involved in the secretory process, from the hepatomas and normal liver. We will construct hybrids of the hepatoma cells by fusion of these cells with each other and with mouse hepatoma to test for complementation. We will also develop screening procedures for use in selecting mutants defective in protein secretion. Four types of mutants will be selected for: 1) those that make serum proteins, but fail to secrete these proteins; 2) those that make serum proteins, but are specifically defective in the secretion of serum albumin; 3) those that secrete serum albumin without normal proteolytic processing; and 4) those that fail to synthesize serum albumin but retain other liver functions. These mutants will then be used to elucidate the various steps in protein secretion and in studies on the control of expression of genes specifying secretory proteins. Procedures for production of antibodies specific for the signal sequence of preproalbumin and the pro sequence of proalbumin will be developed. These antibodies will be used in cytological studies and in mutant selection. The results of the experiments outlined in this proposal should provide new insights into the pathway for protein secretion from liver and may prove useful in understanding liver disease caused by or associated with faulty protein secretion.